"It could happen almost any time now. We now have the technological capability to identify Earth-like planets around the smallest stars."

David Latham -Harvard-Smithsonian Center for Astrophysics

Last week a 21st-century miles was reached: Planet hunters discovered the 500th planet beyond our solar system. To be sure, the vast majority are hot, Jupiter-sized planets that would dwarf the Earth and are almost certainly lifeless. Over the past 15 years, the count of these extrasolar worlds, has soared from single digits to the dozens and then into the hundreds. The pace of discovery is now so rapid that the number of identified planets leaped from 400 to 500 entries in just over a year.

The "book" of exo planet discoveries is being compiled and maintained by Jean Schneider, an astronomer at the Paris Observatory, who since 1995 has maintained The Extrasolar Planets Encyclopaedia of known exoplanets as well as those that are unconfirmed or controversial. Schneider advises caution about celebrating the 500th milestone as there is no consensus on what is a planet and what is a brown dwarf. In an interview with Scientific American, he says: "We don't know exactly where the planets stop and the brown dwarfs start on the mass scale. In addition, the mass scale is not a good criterion. So there is some fuzziness there."

"I think that radial velocity measurements will provide several hundred to a few thousand planets and no more," Schneider added, discussing his view of future discoveries. "Astrometric measurements, and in particular the GAIA mission, are expected to provide a few thousand planets by astrometry, because they are surveying one billion stars. As for microlensing, if a mission like WFIRST is finally launched in 2020, they could have, say, a few hundred planets. Direct imaging will provide certainly more than one hundred but not more than a few hundred, because with direct imaging you cannot go very far away in the galaxy. And the Kepler mission will provide many, at least several tens, of Earth-size planets in the habitable zone of their parent star.

"The number of planets will increase," he added, "until about 2030 and then begin to stop. Another step will start, which will be to characterize more and more closely these planets. Detect more and more molecules, investigate the climate of these planets, et cetera. Another thing we could eventually do is to make the cartography of the planet—to make a multipixel image, to really see the continents. But this is 2050."

In what seems s lifetime ago, in the summer of 2009, a collaboration between the COROT and HARPS systems detected a rocky expolanet five hundred light years away, a small stone ball less than twice the diameter and about five times the mass of Earth – giving it the same density as our place. Don't imagine any aliens just yet though (or if you do, make them pretty heat resistant) – it orbits only 2.5 million kilometers from its star, sixteen times closer than even Mercury gets. The expolanet's "year" is thus shorter than our day – meaning that even if there is an asbestos-based civilization their economy is utterly devastated by birthdays.

COROT is the COnvection ROtation and planetary Transit satellite, scanning thousands of stars to see the tiny dips in brightness caused by planets. HARPS is the High Accuracy Radial Velocity Planet Searcher, a super-sensitive spectrograph installed on a Chilean telescope to accurately identify how fast a wavelength source moves. Between them, they were able to identify the location of the planet and work out it's orbital radius and speed, thereby working out the mass and size. And with names like that, they probably combine to form Voltron's big brother.

Astronomers appear to be on the brink of discovering a second Earth-like planet, a find that would add fresh impetus to the search for extraterrestrial life. Advances in technology suggest scientists are on the verge of being able to detect the presence of small, rocky planets, much like our own, around distant stars for the first time. The planets are considered the most likely habitats for extraterrestrial life.

Finding a rocky planet with an Earth-like density brings us one step closer to discovering another planet similar to our own. A twin-Earth beyond the solar system could provide the best chance of finding non-microbial life elsewhere in the universe.

Everything we've seen previously has been some Jupiter-like gas giant, a huge ball of not-solid-stuff-like-Earth that's still interesting but – since we don't imagine meeting alien clouds very much – not as exciting. But it isn't the case that space only features balls of gas, it's just that our technology couldn't see anything smaller. Until now.

It's awesome stuff for scientists. Yet another example of how we'll never be bored, how the universe is simply swarming with things waiting for us to detect them.